In recent years, a service has been conducted in which resources which are required to build and operate an information system are provided via the Internet. This service is called, for example, Infrastructure as a Service (Iaas). A technique is known in which a plurality of virtual instances are formed in a single physical server, and the virtual instances are provided to users as virtual host computers (for example, refer to Non-Patent Literature 1). In this technique, each physical server has a function of selecting, starting, and managing a virtual instance which has a performance complying with a request from a user. In addition, in this technique, each physical server has a control function of controlling data communication between virtual instances. This control function interrupts data communication between virtual instances provided to different users, or interrupts access to a storage which is allocated to a virtual instance provided to another user. In addition, the instance refers to a set of resources such as a CPU, a memory, and a storage.